MECHANISM OF THE TRANSFORMATION OF PRESSURE FLUCTUATION PRODUCED BY KARMAN VORTEX SHEDDING INTO NOISE

理论和实验研究了叶片、NACA 0008或NACA 0015在均匀迎面流中的近尾迹以及旋转平板近尾迹中产生的噪声与速度波动特性密切相关。结果表明：(1)在较厚叶片NACA 0015的情况下，攻角θ小于约5度时产生离散频率噪声。在这种情况下，压力侧的流动在叶片尾缘上游分离并产生卡门涡街，这是产生离散频率噪声的原因。(2)对于较薄的叶片NACA 0008，当θ =0时，由于没有逆压梯度强烈的位置，DFN不会发生。然而，如果存在一定的攻角，即使攻角很小，由于其相对尖锐的几何形状，气流也会在叶片前缘附近分离。然后产生离散频率噪声。(3)在NACA 0008的情况下，叶片周期速度波动实际上是在离叶片尾缘较远的地方产生的，而不发生DFN。其原因是附在叶片尾缘的气流。这一事实清楚地表明，近尾迹存在较大的死流区是产生DFN的必要条件。(4)尽管来流与旋转流场一样在跨方向上被剪切，但仍会发生DFN。这意味着DFN的产生机制与均匀主流中的机制基本相同。(5)旋转平板噪声频谱的宽带性可以用每次卡门涡脱落引起的周期性速度波动局部产生音调噪声并在叶跨方向上进行积分的物理模型从理论上解释。

It is theoretically and experimentally investigated that the generated noise is closely related with the characteristics of the velocity fluctuation in the near wakes of a blade,NACA 0008 or NACA 0015 placed in a uniform oncoming flow,and in the near wake of a rotating flat plate. The results are summarized as follows:(1)In the case of a thicker blade,NACA 0015,the discrete frequency noise is generated if the attack angle theta is less than about 5 degree. The flow on the pressure side, separates in this case just upstream of the trailing edge of the blade and the Karman vortex street generates,which is the cause to generate the discrete frequency noise. (2)In the case of a thinner blade,NACA 0008,the DFN does not occur if theta=0,because there is no location where adverse pressure gradient is intense. However if there is some attack angle,even if it is small,the flow separates near the leading edge of the blade due to its relatively sharp geometry. Then discrete frequency noise generates. (3)In the case of NACA 0008 blade periodic velocity fluctuation is actually generated relatively far from the blade trailing edge,the DFN,however,does not occur. The reason of this is the flow attached up to the blade trailing edge. This fact shows clearly that the necessary condition of the generation of DFN is the existence of a large dead flow region in the near wake. (4)Although the oncoming flow is sheared in the span direction as in the case of the rotating flow field,DFN occurs. This means the mechanism of the generation of DFN is fundamentally the same that in the uniform main flow. (5)The broad band nature of the spectrum of noise generated from the rotating flat plate can be theoretically explained by the physical model in which the tonal noises are generated locally by the periodic velocity fluctuations due to each Karman vortex shedding and they are integrated in the blade span direction.